1. Field of the Invention
This invention relates generally to a semiconductor bulk acoustic resonator, and more particularly, to a temperature stable semiconductor bulk acoustic resonator incorporating a heating element and a temperature sensor.
2. Discussion of the Related Art
Semiconductor bulk acoustic resonators (SBAR), known in the art, are generally thin film acoustic resonators fabricated on a semiconductor wafer. Typically, the SBAR will include one or more thin layers, or films, of a piezoelectric material, such as zinc oxide or aluminum nitride, with conducting thin film electrode layers above and below the piezoelectric layers. The thin film piezoelectric layers are formed, typically by a sputtering process, on a suitable substrate. The layered structure has an acoustical resonance for acoustic waves traveling perpendicular to the layers, and this resonance appears as an electrical resonance between the electrode layers. SBARs can be configured in a variety of ways, including one-port resonators similar to conventional bulk crystal resonators, and also two-port bandpass filters. For a review of at least one fabrication process to produce such a resonating device, see Cushman, D. et al., "SBAR Filter Monolithically Integrated with HBT Amplifier," Proceedings 1990 IEEE Ultrasonics Symposium, herein incorporated by reference.
One of the advantages of an SBAR is realized from the result that they are fabricated on a semiconductor wafer, and thus, can be integrated into semiconductor circuits, such as oscillators, filter-amplifiers, receivers, etc. Some of these applications, such as oscillators, require very high center frequency stability of the resonating acoustical signal over a wide temperature range. As is known, variances in temperature will generally cause a particular piezoelectric material under a constant driving current to resonate at varying frequencies. Practical frequency stability may require a -20 ppm frequency tolerance over a temperature range from -50.degree. C. to +70.degree. C. Aluminum nitride films, as are presently used in the art, have at best about a 26 ppm/deg. C. temperature sensitivity, and thus are not effective for many of these applications.
In order to maintain an SBAR at a very rigid operating frequency over a wide temperature range, it is known to place the entire chip or resonator assembly in an oven being maintained at a constant elevated temperature. By maintaining the oven at a constant temperature above the maximum ambient temperature expected, the temperature of the resonator is kept constant regardless of the ambient temperature. Therefore, the resonant frequency of the SBAR is stable over changes in ambient temperature. However, these ovens suffer the drawbacks of being exceedingly large, and as a result they consume large amounts of power. Consequently, these drawbacks limit the applicability of highly stable SBARs.
One method of producing an SBAR which is temperature stable in frequency has been proposed in the article by Wang, J.S. et al., "Low Temperature Coefficient Shear wave Thin Films for Composite Resonators and Filters," IEEE Ultrasonic Symposium, 1983. In that article it is proposed to utilize a composite of ZnO/Si or AlN/Si to produce a resonator having a low temperature coefficient. More particularly, shear waves are generated by either a ZnO or a AlN film having their Z-axis oriented from normal to produce quasi-shear waves in these films and pure shear waves in a silicon membrane substrate. This material approach to temperature stabilization is believed to be impractical because of the requirements for extreme accuracy in the film thicknesses and quality.
What is needed then is an SBAR which operates at a very stable frequency over a wide range of ambient temperatures without requiring the SBAR to be placed in a heating oven or requiring impractical piezoelectric film tolerances. It is therefore an object of the present invention to provide such an SBAR.